Semiconductor reliability testing often requires applying a common voltage stress signal to a plurality of devices to gather information about failure times. In order to provide a cost effective and space efficient solution, semiconductor reliability test equipment vendors will often configure their system to have a single voltage source (e.g., a programmable power supply), which is connected to a plurality of DUTs to provide the common stress signal. FIG. 1 illustrates a simplified block diagram of a typical voltage stress system 100. In the most basic form, the plurality of DUTs 102 are all connected directly to the single voltage source 104 and stressed in parallel as shown in FIG. 1. However, this configuration is problematic when a DUT 102 fails because there is no way to disconnect it from the single voltage source 104 and current will continue to flow through the failed DUT 102, which may have failed to a very low resistance and may conduct excessive current loading the single voltage source 104 and/or overheating the failed DUT 102.
An improvement on the above configuration is shown in FIG. 2, in which the modified voltage stress system 200 includes switches 202 in series between the single voltage source 104 and plurality of DUTs 102. This allows a failed DUT 102 to be removed when that failure is detected by the system (e.g., by using an ammeter connected inline with the failed DUT) so the failed DUT 102 does not continue to overload the single power supply and does not overheat. However, this improvement is still problematic because the time between detection of the failure and switching off of the stress signal to a failed DUT may be long enough to disturb the other DUTs, which have not failed yet. Further, the failure event may cause a disturbance in the single voltage source 104 output due to an increase in current when a DUT fails, and this may trigger early failure in the other DUTs.
To avoid the problem described above a further modified voltage stress system 300 is shown in FIG. 3 and includes a current limiter 302 in the signal path of the voltage source 104, switch 202, and DUT 102, as shown in FIG. 3. This current limiter prevents a failed DUT 102 from overloading the voltage source 104 for the period of time between the DUT failure, its detection, and its subsequent removal from the stress signal. The current limiter 302 may simply be a fixed value resistor, or a more complex circuit using active devices to present a non-linear impedance, which is activated (limits) at a specific current level.
The type of configuration presented above is described in U.S. Pat. No. 5,880,540 to Bessho et al., entitled “SWITCHING APPARATUS WITH CURRENT LIMITING CIRCUIT.” However, this configuration is still problematic for a number of reasons. First, the addition of a current limiter circuit and switch adds significant size and cost requirements to the solution. Additionally, the voltage at the DUT is often unknown due to voltage drop across the inline current limiter, and finally, removal of the DUT from the stress signal using a switch 102 may cause transient signals (or “glitches”) to be presented to the remaining DUTs due to the sudden removal of current flow to the failed DUT.